This invention relates to dry pipe valve fire protection sprinkler systems in which the lines leading to the sprinklers are normally isolated from a source of water under pressure by a dry pipe valve which is released in response to detection of a fire hazard.
A conventional dry pipe valve system is disclosed in the Juliano U.S. Pat. No. 3,589,445, the disclosure of which is incorporated herein by reference. In such systems, a plurality of sprinkler heads are mounted on a sprinkler pipe which normally contains air under pressure and which is isolated from a water pipe containing water under pressure by a dry pipe valve. When a sprinkler head is actuated by an ambient temperature above the release temperature of the valve, the dry pipe valve opens and allows the water to flow to the sprinklers. In order to open the dry pipe valve in a more timely manner, an accelerator valve responsive to the pressure drop in the sprinkler pipe is opened to apply air pressure to an intermediate chamber in the dry pipe valve so as to increase the pressure tending to open the dry pipe valve. Conventionally, the air pressure normally applied to the sprinkler line, and to the intermediate chamber when the accelerator is actuated, is at least 20 psi to about 50 psi and the air/water trip ratio, i.e. the ratio between the air pressure in the dry pipe line and the water pressure applied to the dry pipe valve which is required to open the dry pipe valve when the air pressure drops, is about 1:6. When the dry pipe valve has been opened, the valve clapper is latched in the open position and, in order to reset the dry pipe valve, the valve housing must be opened and the clapper must be unlatched manually. In such conventional dry pipe valve systems, the dry pipe valve housing contains several inches of priming water above the valve clapper to aid the sealing process and to preserve the rubber gasket in the valve from detrimental effects produced by the high levels of oxygen in the high pressure air in the dry pipe line.
The Meyer et al. U.S. Pat. Nos. 5,295,503 and 5,439,028 disclose a sprinkler pipe valve which is convertible between a wet configuration and a dry configuration. That valve includes a clapper and a control mechanism for positioning the clapper. In one position, the control mechanism latches the clapper in the closed position to seal the water line from the dry pipe and then permit release of the clapper when the system pressure, which is normally maintained at about 20 psi, drops to a selected level, such as about 15 psi, so as to permit the water to fill the sprinkler line. In this system, the control for the clapper release mechanism utilizes a spool valve which receives the system air pressure at one end and has a balancing spring at the other end so that, when the system pressure drops below the selected value, the valve spool moves in such a way as to release the mechanism that retains the clapper in position. This control mechanism can also reset the clapper to the closed position without requiring the valve housing to be opened. Moreover, when the system has been reset and the valve clapper is closed, all of the water is drained from the valve housing so that no priming water is present above the clapper.